The present invention relates to an improved electromagnetic metering valve for a fuel injector, in particular for internal combustion engines.
The metering valves of fuel injectors normally comprise a control chamber with a discharge conduit, which is normally kept closed by a shutter. In known metering valves, the shutter is normally kept closed by the armature of an electromagnet, with the aid of a spring, and is released to open the conduit by energizing the electromagnet to move the armature towards the core of the electromagnet.
The armature of known valves is normally connected rigidly to a stem sliding in a guide, and, when closing the discharge conduit, the kinetic energy of the armature and stem is dissipated in the impact of the shutter against the valve. More specifically, in the case of a ball shutter, the kinetic energy is dissipated, via a guide plate and the ball, in the impact against the seat in the valve body. Conversely, when opening the discharge conduit, the kinetic energy due to the return movement of the armature and stem is dissipated in the impact of the stem against a stop.
Such impact generates considerable force, which is proportional to the mass and velocity of the armature and stem, and is inversely proportional to impact time, which is very short. Owing to the hardness of the ball and valve body, the impact when closing the valve results in considerable rebound, which is also generated, when opening the valve, by the impact of the stem against the stop. As such, the movement of the armature, and hence the opening and closing movement of the valve, fails to provide for steady operation of the injector.
Moreover, the armature moves inside a fuel discharge chamber in which the pressure and density of the fuel vary considerably. An increase in the pressure, and hence in the density, of the fuel inside the discharge chamber reduces the velocity of the armature both when opening and closing the valve. Finally, the movement of the armature itself produces, and is seriously affected by, pressure waves in the fuel inside the discharge chamber, thus further impairing steady operation of the injector.
A metering valve has been proposed wherein the armature is made of magnetic material; the stem is made of nonmagnetic material to reduce cost, and is detached from the armature to simplify assembly; and the stem is guided by a sleeve forming one piece with a bell fixed to the injector body, so that the stem is arrested by a very small surface and subject to rebound.
Moreover, the armature, stem and guide are assembled together with the electromagnet, so that the travel of the stem cannot be determined without also assembling the armature and the electromagnet, and cannot be measured easily for test purposes.